1. Field of the Invention
The invention relates to a centrifugal clutch in an automotive powertrain and a method for controlling clutch engagement.
2. Background Art
In the design of an automated heavy-duty vehicle powertrain system with an internal combustion engine and a multiple-ratio transmission, it is known practice to include a centrifugally operated clutch for coupling the engine to the torque input side of the transmission. The centrifugally operated clutch functions as a master friction clutch that is engageable during vehicle launch. The engagement is a function of throttle position and other system variables, such as engine speed, transmission input shaft speed, transmission output shaft speed, and engine torque. The transmission typically is a multiple-ratio transmission in which ratio changes are controlled using an electronic microprocessor controller. The transmission may be controlled manually, however, by the operator.
The engine may include a controller for regulating fuel supply to the engine based on a closed loop control technique using a microprocessor controller whereby the controller provides a target engine speed and a target engine torque. In the case of a spark ignition engine, spark timing can be used to achieve momentary torque delivery interruption during transmission ratio shifts. In the case of a diesel compression ignition engine, momentary torque interruption can be achieved using engine fueling control during transmission ratio shifts.
Centrifugally operated clutches in powertrains of this type are well known in the prior art. They typically include centrifugally actuated weights that are rotatable with a driving member coupled to the engine. The weights move freely outward under the effect of centrifugal force to cause the driving member to frictionally engage a driven power output member. Examples of a centrifugally operated clutch of this type can be seen by referring to U.S. Pat. Nos. 6,502,476; 5,441,137; 5,730,269; and 4,610,343.
In heavy duty vehicle powertrains, acceptable vehicle launch performance is achieved using closed loop control of engine speed variables. The clutch remains engaged as the controller allows dynamic shifting. The clutch remains engaged at engine speeds greater than the highest expected speed at which downshifts are initiated. It remains engaged also at engine speeds that are greater than the minimum expected engine speed after an upshift. This is accomplished simultaneously with control of engine fueling of the engine during a launch thereby causing the engine speed and torque to equal or not exceed target values. These are determined as a function of sensed input signals, which may be throttle position, engine speed, engine torque, transmission input speed, transmission output speed, transmission ratio, and clutch slip.
Control of the vehicle engine in a heavy duty powertrain of this type is successful if the engine is calibrated to accept engine speed limit requests from the vehicle controller. The control functions discussed in the preceding paragraphs are not compatible with control of a heavy-duty powertrain of known design that does not include an engine calibrated to respond to engine speed requests. Even if the engine is designed to accept engine speed requests, it may not be calibrated for all vehicle applications that might be used with a transmission with a centrifugally actuated clutch. A control response to a speed limit request is needed to achieve a smooth launch. If an engine of this type is used with a transmission with a centrifugally actuated clutch that does not respond to a clutch control strategy using speed limit requests, the clutch will function poorly and a launch will be characterized by undesirable inertia torque disturbances as the friction surfaces of the centrifugal clutch are engaged.